Few things to check:

• Make sure that the subreport uses the same data source as the main report and the parameter value is correct.
• Check if the height of the detail band in the main report is greater than or equal to the height of the subreport. Also sounds like the height of the subreport should be equal to the height of the frame.
• If the subreport only needs the detail band, all other bands can be removed to make sure its not getting truncated.
• You could also check Print When expressions in both reports to make sure there's no conditional logic preventing them from displaying.